This invention relates generally to projection television screens, and, more particularly, to projection television screens for use with a coherent projection beam.
Rear projection screens transmit an image projected onto the rear of the screen to an audience space and accordingly are sometimes referred to as transmission screens. Rear projection screens typically include a system to diffuse light transmitted into the audience space. One such system includes a plurality of minute colloidal particles. However, when screens with minute colloidal particles are used with high magnification systems in which the projection beam is coherent, a disturbing artifact in the form of a speckle pattern is often observed. Typically, this speckle pattern is more pronounced in screens with high resolution.
Speckle reduction techniques include reducing a coherence of an illumination beam to facilitate a reduction in the visibility of speckle. One way to reduce the coherence is to move one diffusing screen with respect to another diffusing screen to separate the diffusing surfaces as described in S. Lowenthal et al., J. Opt. Soc. Am., pp. 847-851 (1971); N. George et al., Opt. Commun., pp. 71-71 (1975); E. G. Rawson et al., J. Opt. Soc. Am., pp. 1290-1294 (1976) and L. G. Shirley et al., J. Opt. Soc. Am. A, pp 765-781 (1989). Another known way to facilitate a reduction in speckle utilizes a rear phase grating surface as described in U.S. Pat. Nos. 5,760,955 and 6,147,801.